For 250 years it has exercised the minds of theologians, historians and scientists. Charles Darwin was observed pondering its meaning, and Josiah Wedgwood spent many an hour attempting to decipher its cryptic inscription. Some hope it may hold the secret of the whereabouts of the Holy Grail.

Books have been written, documentary films made and poems penned in an attempt to explain it, but the mystery contained in an 18th century monument in the grounds of Lord Lichfield's estate in Staffordshire has eluded interpretation.

In the hope of succeeding where so many others have failed and finally cracking the conundrum, a group of veteran codebreakers from Bletchley Park arrived at Shugborough House near Milford yesterday, armed with proven grey matter and years of experience in deciphering the German enigma codes.

Tucked away within the 900-acre grounds, they found their puzzle: a stone monument built around 1748, containing a carved relief of Nicholas Poussin's Les Bergers d'Arcadie II in reverse. The picture shows a female figure watching as three shepherds gather around a tomb and point at letters within an inscription carved upon it, which read: Et in Arcadia Ego! (And I am in Arcadia too.) Beneath it the letters O.U.O.S.V.A.V.V. are carved, and underneath them a D and an M.

The staff at Bletchley Park, called in to cast an expert eye upon the monument, could not resist the challenge and turned to some of the surviving members of the team who had spent the second world war deciphering codes.

Viewing it for the first time Oliver Lawn, 85, one of the former employees of Bletchley Park, had no doubt that there was a secret to unravel, contained both in the picture and the inscription beneath, and probably based upon missing letters from classical texts.

Mr Lawn, a Cambridge maths graduate who was among the first civilians to be recruited to Bletchley in 1940, deciphered more than 5,000 German codes during the war, using the enigma machine.

He and his colleagues helped to divert German bombers from British cities by breaking the codes that set the radio beams the Nazis used to lead their planes to the target. The successes of the decipherers is thought to have shortened the war by two years.

Their work was so secretive that it was not until recently that Mr Lawn's wife, Sheila, another Bletchley veteran, discovered what his role had been.

But while Mr Lawn normally succeeded in cracking the German wartime codes, he believes the enigma of Shugborough's monument will not be unravelled easily.

"It is totally different in terms of difficulty to what I used to do during the war," he said. "I think you need classical knowledge as well as ingenuity. This is a language rather than a mathematical code.

"Within its genre I would say it's the most challenging I have ever had to tackle. What we need is a bit more intelligence about the family from the documents held at the estate to try and find a key to breaking this. There is always a key, but if this was a code between two people and only they knew it, it could be almost impossible to decipher."

Over the years there have been a number of theories posited about the meaning contained in the Shepherd's Monument. Chief among these is the belief that the connections of the estate's creators, the Anson family, with the grand masters of the closed society of Knights Templar, and the supernatural myths surrounding the estate - where lay lines meet, rivers cross and UFO spotters regularly gather - are evidence that the carving holds the secret to the Holy Grail.

Other solutions are more prosaic. The current Lord Lichfield's great-grandmother believed the letters represented the lines of a poem from Roman mythology about a shepherdess: "Out of your own sweet vale Alicia vanish vanity twixt Deity and man, thou shepherdess the way."

There is always the possibility that the letters mean very little. Richard Kemp, the estate's general manager, said: "They could of course be a family secret, which everyone in the family knows about and which is of little consequence. But it's like Everest, you climb it because it's there. There's a code here, so everyone wants to unravel it."